1. Field of the Invention
The present invention relates to an optical waveguide device for use as a wavelength converter or a second harmonic generator for converting fundamental light into a second harmonic wave having the wavelength which is half that of the fundamental wave, and more particularly to an optical waveguide device for use as a light source for applying a short-wavelength coherent light beam to an optical disc to record information thereon at a high recording density.
2. Description of the Related Art
Optical waves that can be guided in the optical waveguide are limited to an intrinsic mode having discrete propagation constants and can be distinguished by modal orders 0, 1, 2, . . . In general, nth-order intrinsic mode has n nodes in its electric field distribution and an intensity distribution of a far field pattern of light emitted from the waveguide has n nodes accordingly. The existence of nodes is very disadvantageous particularly for devices which focus the emitted light. Therefore, it is customary that optical waveguide devices are designed to utilize a 0th-order waveguide light in which no nodes exist. Wavelength converters using an optical waveguide, for example, are known as one of such optical waveguide devices. In this case, the above-mentioned characteristic imposes a large restriction on designing the devices.
There have been known optical waveguide devices for use as wavelength converters or second harmonic generators for converting fundamental wave having a wavelength .lambda. into a second harmonic wave having a wavelength .lambda./2. The conventional optical waveguide devices are typically classified into three types depending on how the phase matching condition is achieved to generate the second harmonic wave efficiently.
The optical waveguide devices in the first category are known as optical waveguide devices of the guided mode type. In the optical waveguide devices of the guided mode type, phase matching is accomplished by bringing the effective refractive index of a fundamental guided mode and the effective refractive index of a second harmonic guided mode into agreement with each other by using modal dispersions. Nonlinear optical materials which satisfy such a condition are limited, and there have been known no optical materials that would practically achieve a high conversion efficiency in the optical waveguide devices of this group. Also, even when the phase-matching condition is satisfied, a resultant second harmonic wave becomes a high-order waveguide mode of first-order or greater in most cases and is not suitable for being focused in use.
The second class of optical waveguide devices is referred to as quasi-phase matching (QPM) optical waveguide devices which achieve quasi-phase matching by periodically inverting the spontaneous polarization of a nonlinear optical material. The QPM optical waveguide devices are advantageous in that there is available a wide range of optical materials to choose from relatively freely, and a high conversion efficiency can be expected and the 0th-order mode can be used. However, since they require a minute periodic inverted structure to be formed in themselves, a strict control process is needed in the fabrication of the QPM optical waveguide devices. It is difficult to manufacture the QPM optical waveguide devices with good reproducibility with the present fabrication technology.
The optical waveguide devices which belong to the third kind are called Cerenkov-radiation optical waveguide devices that convert a fundamental guided mode into a second harmonic radiation mode. The Cerenkov-radiation optical waveguide devices are advantageous in that the phase matching condition therefor is much looser than those for the optical waveguide devices of the first and second types, and they are simple in structure. However, their conversion efficiency is comparatively low, and they fail to produce a high second harmonic optical output power. Another problem of the Cerenkov-radiation optical waveguide devices is their poor focus characteristics because of a specially shaped pattern of radiation.
As described above, there is available only a limited range of optical materials for the optical waveguide devices which achieve phase matching of the guided mode type. Presently reported examples of such materials are as follows:
"Applied Physics Letters", vol. 24, p. 222.about.(1974), shows an optical waveguide device comprising a linear thin-film waveguide of TiO.sub.2 formed on a nonlinear optical crystalline substrate of quartz. The optical waveguide device converts a fundamental wave having a wavelength of 1.06 .mu.m in a TE.sub.0 (0th order) into a second harmonic wave in a TE.sub.0 mode (0th order). Since the nonlinear optical coefficient of the substrate is very small, the second harmonic wave has an output power of about 10 mW when the fundamental wave has an input power of 100 W. Therefore, the optical waveguide device has a conversion efficiency of only about 10.sup.-6 % and is not particularly usable. This article does not report the generation of a second harmonic wave having a wavelength of blue region from a fundamental wave having a wavelength of 1.0 .mu.m or shorter.
"Optics Communications", vol. 15, p. 104.about.(1975), discloses another optical waveguide device comprising a nonlinear thin-film waveguide of ZnS formed on a nonlinear substrate of LiNbO.sub.3. The optical waveguide device converts a fundamental wave having a wavelength of 1.1 .mu.m in a TE.sub.0 mode into a second harmonic wave in a TE.sub.2 (2nd order). However, the conversion efficiency is very low as the second harmonic wave has an output power of about 5 .mu.W when the fundamental wave has an input power of 70 W. No blue light is produced as the second harmonic wave. Also, the resultant second harmonic wave is in the second-order mode.
"Journal of Crystal Growth", vol. 45, p. 355.about.(1978), discloses still another optical waveguide device comprising a nonlinear thin-film waveguide of LiNbO.sub.3 formed on a linear substrate of MgO. The optical waveguide device converts fundamental waves having respective wavelengths of 1.06 .mu.m and 0.86 .mu.m in a TM.sub.0 mode into second harmonic waves in a TM.sub.1 mode (1st order) and a TM.sub.2 mode (2nd order). However, the conversion efficiency is also very low as the second harmonic wave has an output power of about 1 .mu.W when the fundamental wave has an input power of 10 mW, and only a second harmonic wave of high-order can be obtained.
The reported optical waveguide devices which generate a second harmonic wave of the same polarization by phase matching of the guided mode type have an impractical low conversion efficiency. Their conversion efficiency at the time they convert a fundamental wave into a second harmonic wave in a short wavelength range, such as blue light, is extremely low. Further, the second harmonic wave becomes a high-order mode and nodes are produced in the intensity distribution of emitted light. Therefore, the above optical waveguide device is not suitable for use as the wavelength converter.
For the above reasons, almost no optical waveguide devices of the guided mode type have subsequently been reported so far. Rather, research efforts have been directed to optical waveguide devices which achieve phase matching of the QPM type that can generate a second harmonic wave of a 0th-order mode because of greater freedom for the selection of available optical materials.
A present theme assigned to the QPM type is to manufacture devices with excellent reproducibility.
As described above, in the conventional optical waveguide device, since a guided light of high-order mode has nodes corresponding to orders in the intensity distribution of the emitted light, such nodes are large restriction in designing a wavelength converter which is one of the examples to which the optical waveguide devices are applied.